Method and circuit arrangement for displaying graphics in a digital television receiver

ABSTRACT

The invention relates to a method for displaying graphics in a digital television receiver. A data stream which is provided with video data in the digital format is provided. A first YUV signal having a first frequency screen in the digital format is generated from the video data. The first YUV signal is converted into a second YUV signal having a second frequency screen in the analogous format. An RGB signal having a third frequency screen in the digital format is provided Said RGB signal is provided with graphics data. A mixed signal is provided by means of which the second YUV signal and the RGB signal are mixed together.

[0001] The invention relates to a method for displaying graphics, specifically on a digital television receiver according to claim 1 of the patent, and a circuit for working the method according to claim 8.

[0002] Highly-integrated chip sets are available for digital television sets, these chip sets including a digital TV decoder. The digital TV decoder has an MPEG video decoder, a unit for outputting decoded video data, and an OSD unit (on screen display unit). The OSD unit has the function, for example, of displaying settings for the television set such as volume, color, contrast, brightness, etc. on the screen, but in principle may be used to display any type of information such as data from the Internet. The OSD unit retrieves the graphics data in the same synchronization raster and pixel raster as the video data from a memory, which stores both video data and graphics data, and displays these data on the television screen.

[0003] If, on the other hand, an application requires independent output of graphics and video data, complex circuits must be provided since, for example, in a 100 Hz television receiver, graphics data are not supposed to pass through an algorithm for raising the refresh rate to 100 Hz. This would be superfluous for graphics data. Problems also occur in applications, for example, in which a decoded digital video for recording is fed to an analog video recorder in the background. In this case, the video and/or graphics data are supposed to be displayed by the television receiver, while the analog video recorder is only supposed to record the video data. Currently, no integrated chip set is available for such applications.

[0004] The goal of the present invention is to provide a method for displaying graphics in a digital television receiver and a corresponding circuit which avoids the above problems.

[0005] This goal is achieved by a method according to claim 1 and a circuit with the features of claim 8. Preferred embodiments of the method and circuit are the subject of the dependent claims.

[0006] The invention relates to a method for displaying graphics, specifically on a digital television receiver, wherein:

[0007] a data stream is provided which has video data in digital format,

[0008] a first YUV signal with a first frequency raster is generated in digital format from the video data,

[0009] the first YUV signal is converted to a second YUV signal with a second frequency raster in analog format,

[0010] an RGB signal is provided with a third frequency raster in digital format, which signal contains graphics data, and

[0011] a mixing signal is provided by which the second YUV signal and the RGB signal are mixed together.

[0012] One advantageous aspect of this method, specifically, is the fact that unnecessary conversion of the RGB signal or graphics data to a second frequency raster is avoided. In addition, the method allows for essentially independent processing of the YUV signal and RGB signal, or of the video, data and graphics data, a feature which is required for special applications such as web applications with graphics data and simultaneous transfer of decoded video data for recording by an analog video recorder in the background.

[0013] Preferably, an MPEG decoder receives the data stream and generates from it the first YUV signal. In an especially preferred manner, the MPEG decoder also generates the RGB signal and the mixing signal.

[0014] Preferably, the MPEG decoder provides the YUV signal at a CCIR656 connector in a 50 Hz frequency raster. A conventional converter known from the prior art may be easily connected to the CCIR656 connector for further processing, whereby the converter for example converts the first digital YUV signal to the second analog YUV signal.

[0015] Specifically, the converter converts the first YUV signal to a second analog YUV signal in the 100 Hz frequency raster. Such converters are inexpensive and readily available as standard components.

[0016] Preferably, the third frequency raster of the RGB signal is 100 Hz since the 100 Hz television equipment with digital signal processing has become the standard for the television receiver market.

[0017] The graphics data preferably have twice the resolution of the video data. Since the graphics data include mainly stationary images, not moving images as do video data, the required resolution must be twice that of the video data to achieve a fine, high-resolution rendition of the graphics data.

[0018] The invention further relates to a circuit for displaying graphics on a digital receiver including

[0019] an MPEG decoder to which a data stream is fed which includes video data in digital format,

[0020] a central memory for storing decoded video data and/or graphics data which is readable and writable from the MPEG decoder,

[0021] a video output unit and a graphics output unit to which video data or graphics data are fed from the central memory,

[0022] a mixer to which an output signal from the video output unit and an output signal from the graphics output unit are fed,

[0023] a switch through which the output signal of the graphics output unit is fed to the mixer.

[0024] A circuit of this type essentially implements the method according to the invention and may be easily made in the form of an integrated circuit or chip or chip set.

[0025] Preferably, first horizontal and vertical synchronization signals from the MPEG decoder are fed to the video output unit, while second horizontal and vertical synchronization signals are fed through a multiplexer to the graphics output unit.

[0026] Specifically, a plurality of horizontal and vertical synchronization signals are fed to the multiplexer.

[0027] Preferably, one buffer memory each is placed before the video output unit and/or the graphics output unit, in which memory unit the data retrieved from the central memory are buffered.

[0028] In an especially preferred embodiment, a signal generator is provided which generates third horizontal and vertical synchronization signals and feeds these to the multiplexer.

[0029] Preferably, fourth horizontal and vertical synchronization signals are fed to the multiplexer from an external connector, i.e. for example, through a SCART connector.

[0030] Preferably, the video output unit is operated at a first clock frequency, while the graphics output unit is operated at a second clock frequency. This ensures essentially independent processing of the graphics and video data.

[0031] When the switch is closed, preferably the first and second clock frequencies are the same, and the same horizontal and vertical synchronization signals are sent to the video output unit and graphics output unit.

[0032] Additional advantages and applications may be found in the following description of one embodiment and associated drawings.

[0033]FIG. 1 shows a system for implementing the method according to the invention.

[0034]FIG. 2 is a block diagram of the circuit according to the invention.

[0035]FIG. 1 shows a system which has an MPEG decoder 10 (component SDA 9500) as the central component. A digital data stream 36 is fed to the MPEG decoder 10 by a digital demodulator 31 which serves to demodulate signals modulated by one of the modulation methods QAM, QPSK, or COFDM. Digital demodulator 31 in turn receives digital signals from an analog and digital tuner 30 which receives analog and/or digital television signals.

[0036] MPEG decoder 10 decodes from supplied data stream 36 a digital YUV signal 37 ¹ and provides this signal in 50 Hz format to a CCIR656 output in the form of digital data. The digital data include horizontal, vertical, and synchronization components, as well as a sampling rate of 27 MHz. The YUV signal in 50 Hz format represented by the digital data is up-converted by a converter 32 (component SDA 940x) to the 100 Hz format and is available as the converted and analog YUV signal 39 at one output of converter 32.

[0037] To accomplish these tasks, MPEG decoder 10 has a 32-bit processor core and a graphics memory. In addition, OSD units are provided to process the on screen display unit signal data or graphics data, and a TTX slicer is provided as well.

[0038] In addition, MPEG decoder 10 supplies an analog RGB signal 38 with graphics data at twice the video resolution and an additional mixing signal α, the purpose of which is to mix RGB signal 38 and the converted YUV signal 39. MPEG decoder 10 retrieves the graphics data from an SDRAM (synchronous DRAM) 34 and/or from a ROM/flash 35 in which the data are stored. Mixing occurs in an analog RGB processor which synchronizes the graphics data to the horizontal and vertical components.

[0039] In addition, a first external analog RGB signal 54 may be fed to converter 32, for example through a SCART connector. The supplied RGB signal 54 is then converted to the 100 Hz format for display on the television screen.

[0040] The converted and analog YUV signal 39 is now mixed in an analog RGB processor with RGB signal 38 and, in certain cases, with mixing signal α. A synchronized RGB signal 40 in 100 Hz format is then available at an output of the analog RGB processor 33, which signal may be displayed on the screen of a television.

[0041] A second external analog RGB signal 55 may, for example, additionally be fed through a SCART input to the analog RGB processor 33 and mixed by this processor with analog YUV signal 39 and analog RGB signal 38. Second external analog RGB signal 55 may, for example, be a VGA signal from a computer containing computer data for display on a television receiver.

[0042] Finally, converter 32 may also receive a CVBS signal from analog and digital tuner 30, or from a SCART connector, and convert it to the 100 Hz format. This feature may be used for example to insert a Picture in Picture (PIP) if analog and digital tuner 30 is able to receive in parallel on two different frequencies. Or a signal fed through the SCART connector, such as a signal from a video recorder, may be displayed as a Picture in Picture. Converter 32 may also have the function of image enhancement or digital color decimation.

[0043]FIG. 2 is a block diagram of a system for separate processing of video and graphics data which is based on the method according to the invention. An MPEG decoder 10 is connected bi-directionally with a central memory 11. Central memory 11 may be integrated on one chip as embedded memory together with MPEG decoder 10. Graphics and video data are stored in central memory 11 for display by a television receiver. MPEG decoder 10 writes and reads video and graphics data to/from central memory 11. Both MPEG decoder 10 and central memory 11 are operated at a clock frequency of 108 MHz. This clock frequency is four times the 27 MHz sampling frequency of a video signal.

[0044] Provision is made for a video unit 22 to process the video data and a graphics unit 23 to process the graphics data. Processing of the graphics and video data thus occurs essentially separately in separate units.

[0045] Video unit 22 has a buffer memory 16 (FIFO: First In First Out) for intermediate storage of the video data retrieved from central memory 11. A video output unit 12 follows buffer memory 16, which video output unit processes the video data for display on a television screen. For this purpose, video output unit 12 receives first horizontal and vertical synchronization signals 50 from MPEG decoder 10.

[0046] Graphics unit 23 also has a buffer memory 17 (FIFO: First In First Out) for intermediate storage of graphics data retrieved from central memory 11. A graphics output unit 13 follows buffer memory 17, which graphics output unit processes the graphics data for display on a television screen. For this purpose, graphics output unit 13, like video output unit 12, receives second horizontal and vertical synchronization signals 51 from a multiplexer 14, at the inputs of which in turn first horizontal and vertical synchronization signals 50 from MPEG decoder 10, third horizontal and vertical synchronization signals 52 from an (internal) signal generator 19, and fourth horizontal and vertical synchronization signals 53 from an external source [are applied]². The source of the horizontal and vertical synchronization signals may be set through multiplexer 14.

[0047] Video unit 22 has a mixer 15 following video output unit 12 and a CCIR656 connector 20 following said mixer. A digital YUV signal is applied at CCIR656 connector 20 which must still be converted by a converter to the 100 Hz format. In graphics unit 23, an analog RGB connector 21 is connected after graphics output unit 13 ³, through which RGB connector an RGB signal is output containing the graphics data.

[0048] The output signal of graphics output unit 13 may additionally be coupled through a switch 18 to another input of mixer 15 in video unit 22. The output signal of graphics output unit 13 is mixed together with the output signal of video output unit 12 by the mixer. For this purpose, an additional video/graphics mixing factor a is taken into account which is generated by graphics unit 23.

[0049] The switch enables the system to operate in two modes:

[0050] 1. Switch closed:

[0051] The system operates in a synchronous graphics mode in which video output unit 12 and graphics output unit 13 receive the same horizontal and vertical synchronization signals from MPEG decoder 10. Video/graphics mixing factor a is generated by the graphics unit. The graphics and video data are digitally mixed together.

[0052] 2. Switch open:

[0053] This is the asynchronous graphics mode since graphics unit 23 and video unit 22 are essentially operating independently of one another. Graphics unit 23 may work at any clock frequency independent of the clock frequency of the video unit. The data flow from central memory 11 for graphics display unit 13 is buffered through buffer memory 17. For this purpose, buffer memory 17 receives the clock of central memory 11 as the write clock and the pixel output clock as the read clock. The horizontal and vertical synchronization signals may be derived either from the signal generator 19 internally, for example through counters from the system clock, or from an external source such as a deflection unit. In this mode, graphics unit 23 may feed its data essentially independently from video unit 22 in almost any sampling raster to another component which, based on video/graphics mixing factor α, mixes together in analog form the output signal from CCIR656 connector 20 of graphics unit 22 and the output signal from analog RGB connector 21 of graphics unit 23. 

1. Method for displaying graphics on a digital television receiver, wherein a data stream (36) is provided which has video data in digital format, a first YUV signal (37) with a first frequency raster is generated in digital format from the video data, the first YUV signal is converted to a second YUV signal (39) with a second frequency raster in analog format, an RGB signal (38) is provided with a third frequency raster in digital format, which signal contains graphics data, and a mixing signal (α) is provided by means of which the second YUV signal (39) and the RGB signal (38) are mixed together.
 2. Method according to claim 1, characterized in that an MPEG decoder (10) receives the data stream (36) and generates the first YUV signal (37) from said data stream.
 3. Method according to claim 1, characterized in that the MPEG decoder (10) generates the first RGB signal (38) and the mixing signal (α).
 4. Method according to claims 1 or 2, characterized in that the MPEG decoder (10) provides the first YUV signaI (37) to a CCIR656 connector (20) in a 50 Hz frequency raster.
 5. Method according to one of the foregoing claims, characterized in that a converter (32) converts the first YUV signal (37) to the second YUV signal (39) in a 100 Hz frequency raster.
 6. Method according to one of the foregoing claims, characterized in that the third frequency raster of the RGB signal (38) is 100 Hz.
 7. Method according to one of the foregoing claims, characterized in that the graphics data has twice the resolution of the video data.
 8. Circuit for displaying graphics on a digital television receiver including: an MPEG decoder (10) to which a data stream (36) is fed which includes video data in digital format, a central memory (11) for storing decoded video data and/or graphics data which is readable and writable from the MPEG decoder, a video output unit (12) and a graphics output unit (13) to which video data or graphics data are fed from the central memory (11), a mixer (15) to which an output signal from the video output unit (12) and an output signal from the graphics output unit (13) are fed, a switch (18) through which the output signal of the graphics output unit (13) is fed to the mixer (15).
 9. Circuit according to claim 8, characterized in that first horizontal and vertical synchronization signals (50) are fed to the video output unit (12) by the MPEG decoder, and second horizontal and vertical synchronization signals (51) are fed to the graphics output unit (13) through a multiplexer (14).
 10. Circuit according to claim 9, characterized in that a plurality of horizontal and vertical synchronization signals (50, 52, 53) is fed to the multiplexer (14).
 11. Circuit according to one of claims 8 through 10, characterized in that one buffer memory each (16, 17) is connected before the video output unit (12) and/or before the graphics output unit (13), in which memory data retrieved from the central memory (11) are buffered.
 12. Circuit according to claims 8 through 11, characterized in that a signal generator (19) is provided which generates third horizontal and vertical synchronization signals (52) and feeds these to the multiplexer.
 13. Circuit according to claims 8 through 12, characterized in that fourth horizontal and vertical synchronization signals (53) are fed from an external source to the multiplexer.
 14. Circuit according to claims 8 through 13, characterized in that the video output unit (12) is operated at a first clock frequency, and the graphics output unit (13) is operated at a second clock frequency.
 15. Circuit according to claim 14, characterized in that with the switch (18) closed, the first and second clock frequencies are the same, and the same horizontal and vertical synchronization signals (50) are fed to the video output unit (12) and to the graphics output unit (13). 